Hydrocarbon assets, such as oil and natural gas (“NG”), are often found underground in “tight” geological formations, such as sandstone or shale. These require “unconventional” drilling and completion techniques, including the “fracturing” (or “fracking”) of the geological strata that contain the hydrocarbons to allow those hydrocarbons to be released for recovery, treatment, storage and distribution. Existing fracturing methods are hydraulic, i.e., they use liquids for fracturing and for delivering proppant to the fractures.
However, hydraulic fracturing and proppant delivery methods suffer from a number of significant disadvantages. The liquids that are presently used in standard hydraulic fracturing—for example, chemically modified or treated water at ambient temperatures, and/or cryogenic liquid nitrogen—result in waste streams of contaminated liquid water or gaseous methane containing nitrogen. More particularly, using water or nitrogen results in contamination (or undesirable blending) of both the fracking fluids and the hydrocarbons, and using nitrogen or liquid carbon dioxide requires foaming agents.
The waste streams and contaminated mixtures need to be treated, and the cost of fully cleaning and properly disposing of the “spent” hydraulic fracturing fluid substantially increases the cost of hydraulic fracturing—both in economic terms and environmental terms. If that clean-up is not properly accomplished, the damage of hydraulic fracturing on the environment may be adverse, causing regulators and/or policy-makers to limit the use of hydraulic fracturing in response to concerns by the public at large, as is already the case in some regions today. Hydraulic fracturing also often results in significant methane emissions (with methane being a much more environmentally damaging greenhouse gas than CO2) and may require complex apparatus for mitigating such emissions.
Furthermore, some existing hydraulic fracturing technologies are energy- and capital-intensive. For example, use of liquid nitrogen requires the installation of a plant for air separation that uses deep refrigeration to liquefy ambient air, which is then broken down to yield nitrogen. Using nitrogen for fracking generally requires substantial energy input to achieve the liquid states of the nitrogen. Also, when nitrogen (or more precisely, liquid nitrogen) is pumped to high pressures, as required for the fracturing of deeper formations, a phase shift occurs that shifts the N2 from its liquid foam to its gaseous state, and the delivery of proppant under those conditions becomes problematic.
Proppant often is delivered into fractured subterranean formations by foams because they tend to have lower rates of “leak off” than delivery by liquids, that is, reduced loss of fracking fluid from the fracturing. Most existing proppant delivery utilizes liquid CO2 or liquid nitrogen. However, there are several drawbacks to those techniques such as the transportation costs and logistical complexity of importing the liquid CO2 or liquid nitrogen to the well site, contamination of the hydrocarbons by the liquid CO2 or liquid nitrogen, and the need for water as the liquid base for the foam. The use of water as the liquid base for the foam that is energized by the pumped-to-pressure vaporized CO2 or N2, and which foam delivers the proppant, requires many of the same chemicals (cross linkers, slickening agents, anti-swelling compounds . . . ) as standard hydraulic fracturing.
Accordingly, there is a need for an effective fracturing method that does not use liquids. There is also a need for a more energy-efficient fracturing process. There is a further need for a fracturing method that does not create contaminated waste streams requiring difficult clean-up measures. There is also a further need for a fracturing method that increases the recovery of hydrocarbons from underground formations by avoiding the use of water (which hydrocarbons do not interact well with).
There is also a need for foam-based proppant delivery systems and methods that do not require liquid CO2, liquid nitrogen, or extensive volumes of water. Thus, there is a need for non-hydraulic fracturing and proppant delivery systems and methods which are less energy-intensive, do not require liquids for fracking and proppant delivery, do not significantly add contamination or waste to the fracking process, and have the potential to increase hydrocarbon recovery.